Different wavelengths independent control and functional integration based on a single-layer metasurface

Objective In modern optical systems, the demand for multi-band and multifunctional integrated optical devices is growing increasingly urgent, driven by the rapid development of fields such as dual-band imaging, optical communications, and high-precision laser processing. However, conventional optical systems face inherent limitations in dual-band functional integration: to achieve independent optical regulation at different wavelengths, they typically rely on complex combinations of multiple discrete optical components (e.g., separate lenses, modulators, and filters) or multi-layer composite structures. These designs inevitably result in issues such as large footprint, complicated assembly processes, high optical loss at component interfaces, and potential misalignment between different wavelength channels, which severely restrict the miniaturization, integration, and reliability of optical systems.

Methods A portable target simulation system is built in this paper. A silicon-based metasurface composed of cylindrical nanounits whose phase responses at two target wavelengths are controlled by adjusting their radius (Fig.2). By exploiting the dispersive properties of silicon, the geometric parameters of each unit cell are engineered to independently satisfy phase profiles for both beam shaping and focusing. A systematic optimization process was applied to determine the radius distribution that simultaneously meets the phase-matching conditions for λ₁ = 1064 nm and λ₂ = 1550 nm (Fig.3). Full-wave simulations using the finite-difference time-domain (FDTD) method were performed to evaluate the optical performance and crosstalk between the two functional channels.

Results and Discussions The engineered single-layer metasurface demonstrates exceptional optical performance across two distinct wavelength regimes, successfully implementing fundamentally different wavefront manipulation functions. At 1064 nm, the metasurface operates as a sophisticated beam homogenizer, efficiently converting an incident Gaussian beam into a precisely controlled flat-top beam with a uniformity of 0.1029 and an energy utilization efficiency of 70.32% at the designated transformation plane (z = 55 μm). At 1550 nm, the metasurface acts as a high-numerical-aperture lens, producing a focused spot with a full width at half maximum (FWHM) of 2.52 μm and a focusing efficiency of 65.47% (Fig.4). Critically, the crosstalk between the two functional operations remains below 10% (Fig.5), demonstrating effective decoupling of the two optical functions. These comprehensive results validate the metasurface's capability to perform two fundamentally distinct types of wavefront manipulation, beam shaping and focusing, opening new possibilities for compact, multifunctional photonic systems in applications ranging from dual-wavelength microscopy to integrated quantum photonics and advanced laser processing.

Conclusions Finally, the study proposed and systematically verified a dual-wavelength independent control metasurface based on single-layer silicon cylindrical unit cells. By optimizing the radius parameter of the unit cells and their spatial distribution, the metasurface successfully integrated customized flat-top beam shaping at λ₁ = 1064 nm and diffraction-limited focusing at λ₂ = 1550 nm on a single planar structure, overcoming the defects of complex structure and large volume in traditional dual-wavelength optical systems. Systematic FDTD simulation results confirmed the excellent performance of the metasurface: at 1064 nm, the beam shaping efficiency reached 70.32% with a flat-top uniformity of 0.1029; at 1550 nm, the focused spot had a FWHM of 3.52 μm and the efficiency of 65.47%. Notably, the crosstalk between the two functions was controlled below 10%, ensuring independent and stable operation of each function. This metasurface provides a new technical path for the miniaturization and integration of dual-band optical systems, and has important theoretical reference value and broad engineering application prospects in fields such as dual-band imaging, high-precision laser processing, and optical communication. Future research directions will focus on three key avenues: first, expanding the wavelength regulation range through the adoption of alternative materials, such as titanium dioxide for visible light applications and germanium for mid-infrared regimes; second, optimizing the unit cell structure—for instance, by employing anisotropic geometries—to achieve more sophisticated multifunctional integration, including the seamless incorporation of polarization control capabilities; and third, leveraging advanced micro-nano processing technologies like electron beam lithography to fabricate physical samples and conduct experimental validation of their performance.